Combustion arresters are used to protect people and equipment in many industries such as fuel production, mining, transportation, chemical processing, power generation, and wastewater treatment. For example, combustion arresters may be used to avoid undesired combustion during fuel handling such as in conjunction with storage, transport, consumption, and production of fuel (e.g., natural gas, gasoline, diesel fuel, jet fuel, etc.).
Combustion arresters are installed in systems that interact with gases that may be flammable. Combustion arresters are designed to prevent combustion in one part of the system from igniting nearby flammable gases. Combustion arresters generally include a permeable quenching element enclosed in a housing. The permeable element permits gas to flow but has small passages that are arranged to cool the burning gas of a combustion front to below the autoignition temperature of the gas. Some combustion arresters also may significantly attenuate the pressure wave or shock wave associated with the combustion front.
Design and performance of combustion arresters are dependent on operating conditions, including the type of flammable gas and the temperature, pressure, volume, and flow rate of the flammable gas. Because of the myriad of conditions that may affect performance and the generally unpredictable nature of combustion fronts, a combustion arrester suitable for one situation may not be suitable for another.
Testing of combustion arresters typically involves testing the combustion arrester in the particular circumstances of the expected use. That is, combustion arrester testing generally is a system test (involving actual system components) rather than merely a component test of the combustion arrester. For example, the location of the combustion arrester relative to potential ignition sources and/or flammable gas sources, the size of piping to and/or from the combustion arrester, the type of flammable gas, the temperature and/or pressure of system operation, and the type of ignition sources all may affect the performance of a combustion arrester in a system. To produce reliable results, combustion arrester testing commonly incorporates much, if not all, of the final system components. Especially for large and/or complex installations, reproducing the final design while testing a component of the final design (the combustion arrester) may be a slow and expensive process.
In conventional combustion arrester testing, combustion fronts with different flame propagation conditions are produced (often with as much of the final installation components as possible) to determine if the combustion arrester stops (or does not stop) the particular combustion front. A typical test regime may involve repeating a test several times to verify that the combustion arrester may repeatably stop a combustion front in the given scenario. Such repetitive testing may require rebuilding the test system and/or replacing the combustion arrester for each test run.
Flame speed and combustion front pressure changes may be controlled to some extent by adjusting the length and/or complexity of piping between the combustion arrester and ignition source. Generally, longer pipe lengths lead to faster flame velocities. Additionally or alternatively, a combustion front may be accelerated by introducing specific flame acceleration structures such as a Shchelkin spiral or a series of annular disks. Elbows and tees in the piping also may serve to accelerate a combustion front. Generally, flame acceleration is achieved by increasing turbulence in the gas at the combustion front. Increased turbulence tends to increase combustion, leading to higher pressures (e.g., due to more heating of the burnt gas) and higher combustion front speeds. A combustion front may travel as a deflagration wave (the flame speed is less than the speed of sound in the unburnt gas downstream of the front) or a detonation wave (the flame speed is greater than or equal to the speed of sound in the unburnt gas downstream of the front). In some conditions, a combustion front may transition from a deflagration wave to a detonation wave in what is referred to as a deflagration to detonation transition. During the deflagration to detonation transition, the pressure and flame speed may be much greater than in a detonation wave. The type of combustion front and the flame speed are intimately affected by the particulars of the system design. Additionally, a combustion arrester that may stop an intense detonation wave may not adequately stop a less intense deflagration wave (or vice versa).
As an example of a complicated system, large aircraft typically use combustion arresters in the vent tubes of fuel tanks. To certify and test such configurations, all or most of the aircraft's fuel system may be reproduced (testing in situ). During the design of new systems, the test of the combustion arrester may require finalized components before the design itself is finalized. Further, even successful tests (where the combustion arrester stopped the combustion front) may need to be reproduced (or reproduced under similar conditions) to verify that the performance of the combustion arrester will be reliably successful.